1. Field of the Invention
The present invention relates to a split-flow-type flowmeter for measuring parameters related to flow, particularly flow rate and flow velocity. More particularly, the invention relates to a split-flow-type flowmeter using a temperature-dependent detection element and/or a detection element integrally formed on a semiconductor chip; for example, a split-flow-type flowmeter favorably applicable to a mass flow sensor for use in combustion control of a vehicle or an industrial engine, a mass flow sensor for use in an industrial air-conditioning system or a compressed-air supply system, or a flow sensor for use in control of the air-fuel ratio of a household gas cooker.
2. Description of the Related Art
Japanese Patent Application Laid-Open (kokai) No. 8-5427 discloses a split-flow-type flowmeter to be attached to an intake system of an internal combustion engine. In order to accurately measure flow rate in a main-flow pipe along the regular flow direction (directed from an air intake toward the internal combustion engine), the disclosed split-flow-type flowmeter is designed such that the structure of a split-flow passage, the position of a flow inlet of the passage, and the position of a flow outlet of the passage are determined so as to avoid influence of a pulsating flow; i.e., so as not to detect a pulsation component.
Specifically, the split-flow-type flowmeter disclosed in Japanese Patent Application Laid-Open (kokai) No. 8-5427 is characterized in the following:
(1) The center of an opening of the flow inlet of a flow splitter tube is located at a position on a flow cross section of the main-flow pipe where flow is theoretically of an average flow velocity, for the following reason. When the flow inlet is located at a position where the flow velocity is higher than the average flow velocity (at a position biased toward the center of the main-flow pipe), a measurement error on the plus side arises. By contrast, when the flow inlet is located at a position where a flow velocity is lower (at a position biased toward the wall surface of the main-flow pipe), a measurement error on the minus side arises.
(2) The split-flow passage assumes the shape of the letter L such that a downstream flow path extending on a flow cross section of the main-flow pipe is longer than an upstream flow path extending in parallel with the flow direction in the main-flow pipe, thereby yielding an inertial effect. The inertial effect cancels a measurement error on the minus side which arises from delay in detection response upon occurrence of a pulsating flow in the main-flow pipe.
(3) The flow inlet of the split-flow passage opens on a surface (flow cross section) facing toward a direction opposite the flow direction in the main-flow pipe. The flow outlet of the split-flow passage opens on a surface parallel to the flow direction in the main-flow pipe. Thus, pressure at the flow inlet is always higher than that at the flow outlet, thereby preventing backflow from the flow outlet.
(4) A heating resistor and a temperature-sensing resistor, which constitute a hot-wire flowmeter, are disposed apart from each other in the split-flow passage.
(5) The heating resistor and the temperature-sensing resistor are located within the main-flow pipe.
The split-flow-type flowmeter disclosed in Japanese Patent Application Laid-Open (kokai) No. 8-5427 can measure a flow rate accurately when a flow direction or flow rate remains unchanged for a relatively long period of time; i.e., the flowmeter can measure an average flow rate over a long period of time. However, the split-flow-type flowmeter cannot measure the flow rate of a varying flow with good response. This is because the split-flow-type flowmeter is designed such that the structure of the split-flow passage, the position of the flow inlet of the passage, and the position of the flow outlet of the passage are determined so as not to detect pulsation.
In view of the foregoing, a primary object of the present invention is to provide a split-flow-type flowmeter capable of measuring the flow rate of a pulsating flow. Another object of the present invention is to provide a construction of a split-flow-type flowmeter facilitating achievement of the primary object of the invention. A further object of the present invention is to provide a split-flow-type flowmeter allowing easy optimization of the position of a flow inlet/flow outlet of a flow splitter tube according to the diameter of a main-flow pipe and capable of measuring a flow rate accurately.
To achieve the primary object mentioned above, the present invention provides a split-flow-type flowmeter characterized in that a split-flow passage assumes a flow path structure that is symmetrical with respect to a plane passing through a detection element or with respect to the detection element; a flow inlet and a flow outlet of the split-flow passage open in opposition to each other along a flow direction in a main-flow pipe and are symmetrically positioned with respect to a flow cross section of the main-flow pipe; in order to divert into the split-flow passage a portion of flow having a relatively high flow velocity from the main-flow pipe, the center of opening of the flow inlet/flow outlet of the split-flow passage is positioned between the center of the flow cross section of the main-flow pipe and an average-flow-velocity position at which flow in the main-flow pipe is of an average flow velocity, as observed on the flow cross section of the main-flow pipe.
The split-flow-type flowmeter of the present invention provides the following advantages.
(1) Since the flow inlet and the flow outlet are structured and arranged symmetrically and the split-flow passage assumes the form of a flow path that is symmetrical with respect to the detection element, a regular flow component and a backflow component can be detected with equivalent response.
(2) Since the flow inlet and the flow outlet of the split flow passage open into the main-flow pipe at a position of high flow velocity, the flowmeter can capture a pulsating flow, an abrupt change in flow direction in the main-flow pipe, and abrupt variations in flow rate. Thus, the flow rate can be measured with good response.
(3) Thus, the split-flow-type flowmeter of the present invention can be favorably applied to measurement of a flow rate in the main-flow pipe in which flow may pulsate.
(4) Furthermore, the split-flow-type flowmeter of the present invention can measure, with equivalent response, a regular flow component and a backflow component of a pulsating flow in the main-flow pipe.
The present invention further provides the following constructions of the split-flow-type flowmeter mentioned above. Specifically, the present invention provides a construction of a split-flow-type flowmeter comprising an introduction unit for diverting therein a portion of flow from the main-flow pipe, and a detection unit having a detection element and a flow path peripheral to the detection element and in which, with the introduction unit and the detection unit attached to the main-flow pipe, flow paths belonging to the respective units communicate with each other directly or indirectly to thereby establish the split-flow passage. The present invention further provides a construction of a split-flow-type flowmeter comprising an introduction unit inserted into the main-flow pipe and having a flow inlet for diverting therein a portion of flow from the main-flow pipe, a flow outlet for discharging the portion of flow, and a first flow path communicating with the flow inlet and the flow outlet that is symmetrical with respect to a predetermined plane; and a detection unit having a second flow path that is symmetrical with respect to a predetermined plane and a detection element exposed to flow within the second flow path. Herein, the term xe2x80x9ca predetermined planexe2x80x9d means a plane parallel to a flow cross section of the main-flow pipe as observed in an assembled condition. That is, the first flow path and the second flow path each is symmetrical with respect to a predetermined plane that is parallel to a flow cross section of the main-flow pipe such that a portion of the first/second flow path that is located upstream of the plane with respect to the direction of main flow in the main-flow pipe (with respect to the axis of the main-flow pipe) is symmetrical to a portion of the first/second flow path that is located downstream of the plane with respect to the direction of main flow in the main-flow pipe. The present invention further provides a construction of a split-flow-type flowmeter adapted to attach the introduction unit and the detection unit directly or indirectly to the main-flow pipe via a mounting main-body unit. The present invention further provides a construction of a split-flow-type flowmeter comprising a plurality of units each having a flow path and adapted to attach the plurality of units directly or indirectly to the main-flow pipe to thereby establish communication among the flow paths and thus establish a split-flow passage.
The construction for the split-flow-type flowmeter of the present invention allows selection of an introduction unit whose length corresponds to the diameter of the main-flow pipe such that the flow inlet of the split-flow passage is positioned at a central portion of a flow cross section of the main-flow pipe. Since the detection element and the flow path peripheral to the detection element belong to the same unit; i.e., since the relative position between the detection element and the peripheral flow path is fixed, even when a displacement arises between the introduction unit and the detection unit during assembling work, influence of the displacement on measurement is lessened. Thus, the present invention provides a split-flow-type flowmeter applicable to a main-flow pipe of arbitrary diameter while having a common basic structure, and capable of measuring a flow rate at high accuracy. The present invention also provides a split-flow-type flowmeter which can be adapted not only to the diameter of the main-flow pipe but also to conditions in the vicinity of a measurement position, such as presence of a throttle valve or a bend portion of the main-flow pipe.
First will be described the reason for positioning the center of opening of the flow inlet/flow outlet of the split-flow passage between the center of the flow cross section of the main-flow pipe and a position (hereinafter called the average-flow-velocity position) at which flow in the main-flow pipe is of an average flow velocity, as observed on the flow cross section of the main-flow pipe.
It is self-evident that the flow rate of a near steady-state flow without pulsation can be measured at the average-flow-velocity position. In this case, there is sufficient reason to eliminate the influence of slight pulsation in measurement.
However, the method mentioned above encounters difficulty in measuring an air flow rate accurately when the air flow is in a transient state and involves pulsation including backflow.
A different concept of measurement must be employed; i.e., measuring means capable of capturing a transient state called pulsation must be employed.
In the case of an air flow in the steady state, the flow velocity is the highest at the center of the main-flow pipe and lowers as the point of measurement approaches the pipe wall surface. In the case of an air flow in a transient state, flow is not sufficiently developed in the vicinity of the pipe wall surface due to friction against the pipe wall surface; thus, the difference between a flow velocity at the pipe center and that in the vicinity of the pipe wall surface becomes large as compared with the case of an air flow in the steady state. Also, since flow in the vicinity of the pipe wall surface varies moment by moment due to influence of friction, the flow velocity is unstable.
Accordingly, in the case of air flow involving pulsation, the air flow rate can be measured accurately by measuring at the pipe center, where the air flow develops quickly.
Thus, according to the present invention, in order to divert into the split-flow passage a portion of flow having a relatively high flow velocity from the main-flow pipe, the center of opening of the flow inlet/flow outlet of the split-flow passage is positioned between the center of the flow cross section of the main-flow pipe and the average-flow-velocity position, as observed on the flow cross section of the main-flow pipe. As a result, the state of flow in the main-flow pipe can be captured with good response.
Because of enhanced response capability as mentioned above, the split-flow-type flowmeter of the present invention can measure the flow rate of a pulsating flow, particularly the flow rate of a significantly pulsating flow involving backflow. Accordingly, the split-flow-type flowmeter of the present invention can be favorably used to measure a flow rate in applications involving pulsation; for example, to measure a flow rate for an intake pipe of an internal combustion engine.
A change in flow in an intake pipe with the rotational speed of an internal combustion engine will be schematically described. Referring to FIGS. 13(A) and 13(B) of the accompanying drawings, flow in the intake pipe involves pulsation all the time during transition of the rotational speed of the internal combustion engine from low speed to high speed. During low-speed rotation; i.e., when the time-average air flow rate (represented by the dashed line in FIG. 13(A)) in the intake pipe is low, backflow is generated. During high-speed rotation; i.e., when the time-average air flow rate in the intake pipe is high, pulsation is generated, but backflow is unlikely to be generated.
Next will be described the relationship between the strokes of a 4-cycle internal combustion engine mounted on a vehicle and a detection output of a split-flow-type flowmeter of the present invention in the case where the split-flow-type flowmeter is used to measure a flow rate in an intake pipe of the engine. For comparison, there will be schematically described the relationship between the strokes of a 4-cycle internal combustion engine mounted on a vehicle and a detection output of a conventional split-flow-type flowmeter having a structure to eliminate pulsation and incapable of detecting backflow, such as the aforementioned flowmeter disclosed in Japanese Patent Application Laid-Open (kokai) No. 8-5427, in the case where the split-flow-type flowmeter is used to measure a flow rate in an intake pipe of the engine.
Referring to FIG. 14(A), the split-flow-type flowmeter of the present invention can capture an abrupt rise in flow rate at the initial stage of the intake stroke. When backflow is generated in a period ranging from the last stage of the intake stroke to the initial stage of the compression stroke, the split-flow-type flowmeter of the present invention captures a flow rate on the minus side and issues a minus detection output.
Referring to FIG. 14(B), the split-flow-type flowmeter according to Comparative Example shows slow response in issuing a detection output corresponding to an abrupt increase in flow rate at the initial stage of the intake stroke because of structure to eliminate pulsation. When backflow is generated in a period ranging from the last stage of the intake stroke to the initial stage of the compression stroke, the split-flow-type flowmeter according to the Comparative Example issues a detection output involving a measurement error on the plus side, since the flowmeter cannot identify a flow rate on the minus side characteristically and structurally.
Particularly, the split-flow-type flowmeter of the present invention is favorably employed to measure a flow rate in an intake pipe of an internal combustion engine to be mounted on a two-wheeled vehicle (i.e., motorcycle) or a compact four-wheeled vehicle (i.e., compact car). This is because the intake pipe of an internal combustion engine for use in a two-wheeled vehicle or a compact four-wheeled vehicle is short and involves great fluctuations in flow therein.
As shown in FIG. 15(A), in the case of a two-wheeled vehicle, even when the engine speed is constant, pulsation and even backflow are generated in an intake pipe for the structural reason of the intake system and engine (see the left-hand drawing in FIG. 15(A)); hence, the flow velocity distribution in the intake pipe fluctuates all the time, resulting in significant variations in flow rate in the intake pipe.
As shown in FIG. 15(B), in the case of a four-wheeled vehicle (particularly of medium or large size), when the engine speed is constant, flow in an intake pipe is relatively stable for the structural reason of the intake system and engine (see the left-hand drawing in FIG. 15(B)).
For accurate measurement of the flow rate of a pulsating flow and the flow rate of backflow, a hot-wire flowmeter of quick response employing a detection element formed integrally on a semiconductor substrate of small heat capacity, which will be described later, is preferred.
Preferred, i.e., optional, aspects of the present invention will next be described.
According to a preferred aspect of the present invention, the position of the flow inlet and the flow outlet of the split-flow passage is determined so as to divert into the split-flow passage a portion of flow having a relatively high flow velocity from the main-flow pipe. Preferably, the center of opening of the flow inlet/flow outlet of the split-flow passage is positioned within the radius (a radius on a flow cross section of the main-flow pipe) 5D/16 (particularly within a circular area of a radius 5D/16) from the center of the flow cross section of the main-flow pipe (hereinafter is also called xe2x80x9cthe center of the main-flow pipexe2x80x9d), where D is the diameter of the flow cross section of the main-flow pipe. More preferably, the center of opening of the flow inlet/flow outlet is positioned within a radius D/4 from the center of the main-flow pipe.
Steady State
Generally, it is said that when a flow within a pipe is in the steady state, a portion of the flow having an average flow velocity is located 8-15% the pipe diameter D away from the inner wall surface of a pipe; i.e., 0.35D-0.42D away from the pipe center (refer to xe2x80x9cReissued Handbook of Visualization of Flow,xe2x80x9d Chapter 1 xe2x80x9cBasics of Visualization of Flow,xe2x80x9d 1.2.3 xe2x80x9cLaminar Flow and Turbulent Flow,xe2x80x9d (1) Averages of Momentum, etc., Edited by The Visualization Society of Japan, Published by Asakura Shoten, 6th Issue, Jun. 15, 1995; and xe2x80x9cMechanical Engineering Handbookxe2x80x94Basics,xe2x80x9d Chapter II xe2x80x9cFlow in Flow Path,xe2x80x9d Written and Published by The Japan Society of Mechanical Engineers, 8th Issue, Nov. 28, 1997).
Transient State
In the transient state, air flow is developed slowly in the vicinity of the pipe wall surface due to friction and is developed quickly at a central portion of the pipe. Thus, when, as mentioned above, the center of opening of the flow inlet/flow outlet of the split-flow passage is positioned within a radius 5D/16 from the center of the main-flow pipe, flow of sufficiently high velocity can be diverted into the split-flow passage.
A split-flow-type flowmeter according to a preferred aspect of the present invention is such that L is not greater than D/2, where D is the diameter of the flow cross section of the main-flow pipe and L is the width of opening of the flow inlet/flow outlet of the split-flow passage. Preferably, L is not greater than D/4. Thus, flows of different flow velocities distributed on the flow cross section of the main-flow pipe can be diverted into the split-flow passage, whereby the current state of flow can be captured accurately.
A split-flow-type flowmeter according to a preferred aspect of the present invention is such that a bypass flow path connecting straight the flow inlet and the flow outlet and bypassing the split-flow passage extending on the detection element is formed. Thus, diversion of flow into the split-flow passage extending on the detection element is facilitated and contaminant (particles) flows through the bypass flow path.
A split-flow-type flowmeter according to a preferred aspect of the present invention is such that a portion of the split-flow passage decreases gradually in cross-sectional diameter from the flow inlet toward the detection element and from the flow outlet toward the detection element. Thus, diversion of flow into the split-flow passage extending on the detection element is facilitated.
A split-flow-type flowmeter according to a preferred aspect of the present invention is such that the detection element is disposed on a bottom portion of the split-flow passage and such that the detection element and a flow path extending in the vicinity of inflection portions located before and after the detection element belong to the same unit. Thus, even when the introduction unit is replaced, detection characteristics of the detection element are held unchanged.
A split-flow-type flowmeter according to a preferred aspect of the present invention is such that a recess is formed in the detection unit so as to serve as the second flow path and the detection element is disposed on the bottom of the recess. Preferably, a partition for partitioning the first flow path is disposed within the introduction unit to thereby form the split-flow passage into the shape of the letter U by means of the partition and the first and second flow paths (to thereby form a bent or curved split-flow passage).
A split-flow-type flowmeter according to a preferred aspect of the present invention comprises a main-body unit having a cavity formed therein for accommodating a base portion of the introduction unit and the detection unit and on which a control circuit for controlling the detection element is mounted, and is such that a portion of the main-body unit surrounding the cavity is fixed to the main-flow pipe.
A split-flow-type flowmeter according to a preferred aspect of the present invention is such that the main-body unit or the introduction unit comprises a U-shaped wall curved in such a manner as to enclose a portion of the partition located on the detection element side and adapted to further partition the first flow path.
A split-flow-type flowmeter according to a preferred aspect of the present invention is characterized in that the width of a flow path (first flow path) located away from the detection element is greater than that of a flow path (second flow path) located near the detection element. Preferably, an end portion of a surface of the detection unit defining the flow path located near the detection element (second flow path) is chamfered in a straight line or a curved line.
A split-flow-type flowmeter according to the present invention can measure not only flow rate but also parameters related to flow, such as flow velocity, as needed.
In order to realize stable measurement at high accuracy, a preferred aspect of the present invention comprises a bypass flow path connecting straight the flow inlet and the flow outlet and bypassing the split-flow passage and/or a venturi for decreasing the diameter of the split-flow passage in the vicinity of the detection element. The bypass flow path stabilizes supply of fluid to be measured to the detection element and facilitates diversion of fluid to be measured (flow in the main-flow pipe) into the split-flow passage. The venturi effectively eliminates turbulence of fluid to be measured which would otherwise arise on the detection surface of the detection element. Thus, even when pulsation or pulsation plus backflow is generated, measurement is stabilized and measurement at high accuracy becomes possible.
Particularly, in the case of the split-flow passage assuming the structure of flow path symmetrical with respect to the detection element, employment of an orifice for decreasing the diameter of a flow cross section of the bypass flow path further stabilizes flow reaching the detection element even when pulsation or pulsation plus backflow is generated.
According to a preferred aspect of the present invention, an orifice is disposed in the bypass flow path to thereby determine the flow rate of fluid to be measured and diverted toward the detection element, by means of the amount of projection of a flow path wall of the orifice or the area of opening of the orifice. Thus, the flow rate of flow heading for the detection element can be quantitatively controlled.
According to a preferred aspect of the present invention, means for forming flow hitting obliquely on the detection surface of the detection element is provided in the split-flow passage. The flow control means causes steady flow onto the detection surface of the detection element, so that flow to be detected reliably flows on the detection surface. Additionally, since generation of vortex and separation in the vicinity of the detection surface is suppressed, detection accuracy and reproducibility are enhanced.
According to a preferred aspect of the present invention, flow control means for forming flow hitting obliquely on the detection surface of the detection element or forming flow flowing obliquely with respect to the detection surface assumes the form of a flow path surface (an elevated portion) elevated above the detection surface, which elevated flow path is located upstream of the detection element, or upstream and downstream of the detection element. The form of elevation is not particularly limited so long as flow hitting obliquely on the detection surface is formed. Preferably, the form of elevation is concave or convex, or the elevated surface is a linear, polygonal, or concavely-curved slant surface.
According to a preferred aspect of the present invention, the detection surface of the detection element is exposed to the interior of the split-flow passage (detection tube) at an inflection portion of the split-flow passage. Preferably, the split-flow passage is attached to the main-flow pipe (a pipe at which measurement is performed) in a perpendicularly intersecting condition, and the detection element is disposed at an inflection portion (a bent portion, or a curved portion of flow path) of the split-flow passage. Alternatively, the detection element is disposed at or in the vicinity of a portion of the split-flow passage where flow is inverted or the direction of flow is changed greatly. Preferably, the detection surface of the detection element is exposed to a portion of the interior of the split-flow passage where flow is fast. Preferably, the detection surface of the detection element is exposed to a portion, or its vicinity, of the interior of the split-flow passage where flow is throttled and then changes its direction.
According to a preferred aspect of the present invention, the detection element mounted on the bottom wall of the split-flow passage is located outside the main-flow pipe. Thus, the detection element can be readily mounted or replaced. Also, output from the detection element can be readily led out.
A preferred aspect of the present invention can use the following detection element. Specifically, the detection element is a thermal detection element comprising a semiconductor chip and four thin-film resistors formed on the chip. More specifically, a diaphragm section and a rim section are formed on a semiconductor layer. The diaphragm section includes (1) an upstream temperature sensor, (2) a downstream temperature sensor, and (3) a heater disposed between the upstream temperature sensor and the downstream temperature sensor. The rim section includes (4) an ambient temperature sensor. The diaphragm section is finished very thin and thermally insulated.
Next will be described the principle of detection of flow-related parameters, such as flow velocity and flow rate, by use of the detection element.
(1) Power supplied to the heater is controlled such that a constant difference is maintained between the temperature of the heater and the ambient temperature.
(2) Thus, when flow is not present, the upstream temperature sensor and the downstream temperature sensor indicate substantially the same temperature.
(3) However, when flow is present, heat escapes from the surface of the upstream temperature sensor; thus, the temperature of the upstream temperature sensor drops. Because of an increase in thermal input from the heater, a temperature change of the downstream temperature sensor is smaller than that of the upstream temperature sensor. Notably, in some cases, the temperature of the downstream temperature sensor may rise.
(4) Flow rate, flow velocity, or a like parameter is detected on the basis of the temperature difference between the upstream temperature sensor and the downstream temperature sensor. The direction of flow is detected from the sign of the temperature difference. Notably, the temperature difference can be detected on the basis of a change in electrical resistance caused by temperature.
A preferred aspect of the present invention can use the following other detection element. Specifically, the detection element is a thermal detection element comprising a semiconductor chip and three thin-film resistors formed on the chip. More specifically, a diaphragm section and a rim section are formed on a semiconductor layer. The diaphragm section includes (1) an upstream heater and (2) a downstream heater. The rim section includes (3) an ambient temperature sensor. The diaphragm section is finished very thin and thermally insulated.
Next will be described the principle of detection of flow-related parameters, such as flow velocity and flow rate, by use of the detection element.
(1) Power supplied to the upstream and downstream heaters is controlled such that a constant difference is maintained between the upstream/downstream heater and the ambient temperature.
(2) Thus, when flow is not present, the upstream heater and the downstream heater indicate substantially the same temperature.
(3) However, when flow is present, heat escapes from the surfaces of the upstream and down stream heaters; thus, the temperature of the upstream and downstream heaters drops. Because of an increase in thermal input from the upstream heater, a temperature change of the downstream heater is smaller than that of the upstream heater. Notably, in some cases, the temperature of the downstream heater may rise.
(4) Flow rate, flow velocity, or a like parameter is detected from the difference in current or voltage required to maintain a constant temperature between the upstream heater and the downstream heater as obtained on the basis of a temperature drop of each of the upstream and downstream heaters. The direction of flow is detected from the sign of the current or voltage difference. Notably, the temperature drop can be detected on the basis of a change in electrical resistance caused by temperature.
According to a preferred aspect of the present invention, the detection element measures flow-related values including at least flow rate and/or flow velocity on the basis of temperature.
A split-flow-type flowmeter of the present invention can be installed in an intake system of an engine to be mounted in various kinds of vehicles, two-wheeled and four-wheeled, in order to measure an intake rate or a like parameter. For example, a split-flow-type flowmeter of the present invention is installed in an intake system of an engine to be mounted in a four-wheeled vehicle, somewhere between the interior of an air cleaner and a throttle valve. A split-flow-type flowmeter of the present invention is installed in an intake system of an engine to be mounted on a two-wheeled vehicle; specifically, on an intake pipe (air funnel) connected to a cylinder, in order to measure an intake flow rate, an intake flow velocity, or a like parameter.